The present invention relates to a method for removing a particulate contaminant material from a particulate mixed lithium metal phosphate material.
The use of synthetic mixed lithium transition metal phosphates, especially lithium iron phosphate (LiFePO4), as an alternative cathode material in lithium ion batteries is known from the prior art and is subject to numerous research efforts. This was described for the first time in A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc. Vol. 144 (1997) and is also disclosed, for example, in U.S. Pat. No. 5,910,382.
A number of synthetic methods for obtaining doped and undoped lithium transition metal phosphates have been described so far.
WO 02/099913 A1 discloses a synthetic method, wherein the water is evaporated from an equimolar aqueous solution of Li30, Fe3+ and PO43− to produce a solids mixture, after which the solids mixture is decomposed at a temperature below 500° C. in order to produce a pure mixed Li/Fe phosphate precursor. Phase pure LiFePO4 powder is then obtained by reacting the precursor at a temperature of below 800° C. in a reducing atmosphere.
Also solid state processes are known from the prior art. Drawbacks include firstly the high material costs of the starting chemicals (e.g. iron oxalate). The consumption of protective gas during the sintering process is also considerable, and toxic by-products, such as CO, are formed during sintering. It has also been discovered that the particle size distribution of the product so obtained is often very wide and bimodal. Further production processes are known, for example, from WO 02/083555, EP 1 094 523 A1, US 2003/0124423 and Franger et al., Journal of Power Sources 119-121 (2003), pp. 252-257.
JP 2002-151082 A describes lithium iron phosphate, processes for producing it and a secondary battery which uses it. The process for producing lithium iron phosphate is characterized in that a lithium compound, a divalent iron compound and a phosphoric acid compound are mixed with one another in such a way that at least the molar ratio of the divalent iron ions and the phosphoric acid ions is approximately 1:1, and the mixture is made to react in a temperature range from at least 100° C. up to at most 200° C. in a tightly closed vessel with the addition of a polar solvent and an inactive gas. The lithium iron phosphate obtained in this way can then be physically comminuted.
Although usable lithium iron phosphate can already be obtained using the processes according to the prior art, said production processes nevertheless have the drawback that it is not possible to obtain pulverulent lithium iron phosphate with a very small particle size and a very narrow particle size distribution.
A process for producing mixed lithium metal phosphates, like for example lithium iron phosphate, avoiding said drawbacks of the prior art and in particular providing a material which is especially suitable for electrodes of rechargeable batteries is described in US 2007/0054187 A1.
The process according to US 2007/0054187 A1 is carried out by producing a precursor mixture containing at least one Li+ source, at least one M2+ source and at least one PO43 source, wherein M comprises at least one metal from the first transition series, in order to form a precipitate and thereby to produce a precursor suspension, dispersing or milling the precursor mixture or suspension until the D90 value of particles in a precipitate of the precursor mixture or suspension is less than 50 μm, and obtaining LiMPO4 from the precursor mixture or suspension by reaction under hydrothermal conditions.
The LiMPO4 products obtained according to US 2007/0054187 A1 have satisfactory properties for using them as electrode materials of rechargeable batteries.
However, when using the LiMPO4 products obtained according to US 2007/0054187 A1 in practice it has been noted that lithium ion cells produced with said LiMPO4 products sometimes suffer from an increased self discharge and failure rate. When analyzing the LiMPO4 products used for said lithium ion cells it was found that they included particulate contaminations, in particular metallic and/or oxidic (oxide) particulate contaminations, like for example Fe and Fe oxides in case of LiFePO4, having an average particle size which is above that of said LiMPO4 products. The amount of said particulate contaminations was in a range of 1 ppm to 10 ppm, based on the LiMPO4 product.
In order to lower or preferably exclude the occurrence of an increased self discharge and failure rate of lithium ion cells produced with said LiMPO4 products, there is a need to provide a simple but effective method for removing said particulate contaminations from said LiMPO4 products.
According to a conventional technique for removing particulate contaminations from particulate material, wherein the particulate contaminations have a larger particle size than that of the particulate material, the contaminated particulate material in a fluidized bed is continuously passed through a sifting device, like for example a cyclone or a sifting wheel. According to this technique, the finer particles of the particulate material are separated from coarse particles of the particulate contaminations, which are rejected into the fluidized bed. In case the contaminated particulate material is milled before it is fluidized/sifted, it is also known that hard-to-mill particulate contaminations like metal particles do not pass the sifter as easily as the main product as they stay bigger and can therefore be more or less accumulated in the fluidized bed.
When the above-discussed conventional technique was applied for removing a particulate contaminant material from a particulate mixed lithium metal phosphate material it turned out that the conventional technique was not effective enough in removing a large quantity, preferably essentially all particulate contaminant material from a particulate mixed lithium metal phosphate material.